1. Technical Field
This application was filed claiming Paris Convention priority of Japanese Patent Application No. 2008-287060, the entire content of which is herein incorporated by reference.
The present invention relates to a process for producing methionine by a hydrolysis reaction of 5-[(2-(methylthio) ethyl)]imidazolidine-2,4-dione [refer to the following reaction formula (1)]. Methionine is useful as an additive to feeds for animals.

2. Background Art
The following process is known as one of processes for producing methionine: that is, 5-[(2-(methylthio) ethyl)]imidazolidine-2,4-dione is hydrolyzed under a basic condition, using a basic potassium compound such as potassium carbonate or potassium hydrogencarbonate, to thereby produce methionine. In this process, carbon dioxide is introduced into the reaction solution obtained after the hydrolysis, to thereby cause crystallization, so that methionine is separated and obtained as crystals. The mother liquor obtained after the separation of methionine still contains methionine at a concentration equivalent to a solubility and also contains potassium hydrogencarbonate which can be recycled as the above-described basic potassium compound. This mother liquor, therefore, should be recycled for use in the above-described hydrolysis reaction. In this step, when the entire amount of the mother liquor is recycled, impurities tend to accumulate, and therefore, the mother liquor is needed to be purged of at a given rate. To treat the mother liquor thus purged of as liquid-waste leads to a loss of methionine and potassium hydrogencarbonate contained in the mother liquor, and a cost for the treatment of liquid-waste is considerably high. Therefore, this method is not advisable.
Under such a circumstance, there are reported many methods for recovering, from mother liquors as described above, methionine and potassium hydrogencarbonate as so-called second crystals. For example, the following method is disclosed in JP-B-54-9174: the above-described mother liquor is mixed with a water-soluble solvent such as an alcohol (e.g., methyl alcohol) or acetone; and then, carbon dioxide is introduced into the resulting mixture to cause crystallization. The following method is also disclosed in JP-A-51-1415: the above-described mother liquor is concentrated; and then, carbon dioxide is introduced into the resulting concentrate to cause crystallization. The following method is further disclosed in JP-A-5-320124: the above-described mother liquor is mixed with isopropyl alcohol; and then, carbon dioxide is introduced into the resulting mixture to cause crystallization. The following method is further disclosed in JP-A-2007-63141: the above-described mother liquid obtained after the separation of the first crystals is concentrated; then, the resulting concentrate is subjected to a heat treatment at 165° C.; then, the treated solution is mixed with isopropyl alcohol; and then, carbon dioxide is introduced into the resulting mixture to cause crystallization. This Patent Publication also discloses a method of recovering methionine as the third crystals as follows: the mother liquor obtained after the separation of the second crystals is concentrated; and carbon dioxide is introduced into the resulting concentrate to cause crystallization.